The present disclosure relates to systems and methods for stripping a substrate from an image transfer unit, such as photoreceptors found in printers, photocopiers, facsimile machines and the like.
One type of known printing system or digital imaging system is depicted in FIG. 1. Printing jobs are submitted from a print controller client 10 to a print controller 12. The print controller client 10 may be an electronic copier, printer, facsimile or computer that creates or transmits digital image data. A pixel counter 14 is incorporated into the print controller to count the number of pixels to be imaged with toner on each sheet or page of the job, for each color. The pixel count information is stored in a memory of the print controller 12. Job control information, including the pixel count data and digital image data, are communicated from the print controller 12 to a control unit 20. The digital image data represents the desired output image to be imparted on at least one sheet of a substrate. The control unit 20 may be a microprocessor or other control device.
A photoreceptor surface 26 advances sequentially through various xerographic process stations in the direction indicated by arrow 26. The surface 26 may be a charge retentive surface on a photoreceptor belt, such as an active matrix photoreceptor belt. Other types of photoreceptors, such as a photoreceptor drum, may be substituted for the belt 26 for sequentially advancing through the xerographic process stations. A portion of the photoreceptor belt 26 passes through charging station A, where a charging unit 28 charges the photoconductive surface of photoreceptor belt 26 to a substantially uniform potential. Preferably, charging unit 28 is a corona-generating device such as a corotron.
Subsequently, the charged portion of photoreceptor belt 26 is advanced through imaging/exposure station B. The control unit 20 receives the digital image data from the print controller, processes and then transmits this digital image data to an exposure device 30 located at imaging/exposure station B. The device may be a raster output scanner (ROS) or other xerographic exposure device, such as a plurality of light emitting diodes (an LED bar). The output of the exposure device causes the charge retentive surface of the photoreceptor belt 26 to be discharged at certain locations on the belt in accordance with the digital image data output from the digital image generating device. Thus, a latent image is formed on photoreceptor surface 26.
Next, the photoreceptor surface 26 advances the latent image to a development station C, where toner is electrostatically attracted to the latent image using commonly known techniques. The latent image attracts toner particles contained in a developer unit 36, forming a toner powder image thereon. Alternatively,the developer unit 36 may utilize a hybrid development system in which the developer roll, better known as the donor roll, is powered by two developer fields (potentials across the air gap). The first field is the ac field which is used for toner cloud generation. The second field is a dc developer field which is used to control the amount of toner mass developed on the photoreceptor belt 26. Appropriate developer biasing is accomplished by way of a power supply. This type of system is a non-contact type in which only toner particles are attracted to a latent image and there is no mechanical contact between the photoreceptor belt 26 and the toner delivery device. However, the developer unit 36 may utilize a contact system as well.
Subsequent to image development, a substrate S is moved into contact with toner images at transfer station D. The substrate S is obtained from a supply and advanced to transfer station D by any known sheet feeding apparatus (not shown). The substrate S is then brought into contact with the photoconductive surface of photoreceptor belt 26 in a timed sequence so that the toner powder image developed thereon contacts the advancing substrate S at transfer station D. Transfer station D preferably includes a transfer unit 40. Transfer unit 40 may include a corona-generating device, such as a corotron. The corona-generating device sprays ions onto the backside of substrate S. These ions attract the oppositely charged toner particle images from the photoreceptor belt 26 onto the substrate S. A detack unit 46, such as a detack corotron, is provided for facilitating stripping of the substrate S from the photoreceptor belt 26.
After transfer, the substrate S continues to advance toward fuser station E on a conveyor belt (not shown) in the direction of arrow 44. Fuser station E includes a fuser unit 42, which includes fuser and pressure rollers to permanently affix the image to the substrate S. After fusing, the substrate is the advanced in a known manner to a catch tray, stacker, finisher or other output device (not shown), for subsequent removal from the print engine by the operator.
After the substrate S is separated from photoconductive surface of photoreceptor belt 26, the residual toner particles carried by the non-image areas on the photoconductive surface are removed therefrom. These particles are removed at cleaning station G, using, for example, a cleaning brush or plural brush structure or any number of well known cleaning systems.
Control unit 20 regulates the various print engine functions. The control unit 20 is preferably a programmable controller (such as a microprocessor), which controls the print engine functions. The control unit 20 may provide a comparison count of the copy sheets, the number of documents being recirculated, the number of copy sheets selected by the operator, time delays, jam corrections, etc. The control of all of the exemplary systems heretofore described may be accomplished by conventional control switch inputs from the printing machine consoles selected by an operator.
As is known, as a portion of the photoreceptor belt 26 passes through the charging station A, the charging unit 28 charges the photoconductive surface of the belt portion to a relatively high, substantially uniform potential. This potential is conventionally a negative voltage −V0, which is typically between −600V and −600V. At the imaging station B, the charged portion of the photoconductive surface is exposed to the scanning device 30, which is controlled by the control unit 20 as a function of signals from the print controller 12. The print controller 12 conveys digital signals representing the desired output image that is obtained from the print controller client 10. When exposed at the exposure station B, the photoreceptor surface is selectively discharged to a level of about −60V to −80V. Thus, after exposure, the photoreceptor belt 26 contains a monopolar voltage profile of high voltage, corresponding to charged areas, and low voltage, corresponding to discharged or background areas. This monopolar voltage profile forms the electrostatic latent image.
At the development station C, toner particles are provided that are attracted to the electrostatic latent image. In a known non-contact developer unit, a donor roller is powered by first field, which is an ac field adapted for toner cloud generation, and a second field, which is a dc field used to control the amount of developed toner mass on the photoreceptor surface. At the transfer station G, positive ions applied to the backside of the substrate S by the transfer unit 40 attract the negatively charged toner powder previously applied to the photoreceptor surface 26. In a typical system, the positive ions are generated by a transfer corotron that includes at least one wire, or coronode, which functions to generate electric fields. The necessary electrical field is provided by applying a particular bias to the corotron, which in the case of a transfer corotron is typically a substantially DC voltage or current bias. The actual voltage on the transfer corotron may be changed for different paper types and altitudes, etc., but the transfer current is typically kept constant. The magnitude of the positive transfer voltage may be approximately equal to the lower negative voltage at the imaging station, or between about +60V and +80V.
The detack unit 46 is also typically a corotron to which an electrical bias is applied. A common detack corotron is powered by an alternating current with a DC bias. The detack corotron is operable to generate an electrical field capable of neutralizing the charge on the substrate that attracts the substrate to the photoreceptor surface 26. More particularly, certain detack corotrons deposit both positive and negative ions onto the back of the substrate at the frequency of the line source until the net charge on the back of the sheet rapidly approaches the potentials on the photoreceptor surface 26. Once the potential is neutralized, the substrate tends to separate from the photoreceptor surface, sometimes assisted by a mechanical stripper inserted between the substrate and surface. The magnitude of the neutralizing potential may be approximately equal to the maximum negative potential at the imaging station, or between about
It is known that higher neutralizing charges at the leading edge of the substrate will assist in stripping. On the other hand, it is also known that these higher detacking charges reduce the efficiency of the transfer unit 40, or lead to other undesirable effects such as image washout in the leading edge region or increased instability of the unfused transferred image on the substrate. There is a need for a system and method that can adjust the detacking levels to balance the need to assist in stripping a substrate from a photoreceptor surface and the desire to maintain the print quality as high as possible.